324 Prof. Hclmholtz on the Methods of Measuring 



with the frog. Not only can wc not remove the still unknown 

 influence of the nervous conduction in the brain and the spinal 

 column, but we must actually make use of them in the course of 

 experiment. After, however, having established by rigorous 

 experiments that in the nerves of the frog a sensible time is 

 required for the propagation of an impression, I believe I need 

 not hesitate to indicate the results of the experiments which up 

 to the present time I have made upon the human subject. 



The intelligence of an impression made upon the ends of the 

 nerves in communication with the skin is transmitted to the 

 brain with a velocity which docs not vary in different individuals, 

 nor at different times, of about 60 metres (195 feet) per second. 

 Arrived at the brain, an interval of about one-tenth of a second 

 passes before the will, even when the attention is strung to the 

 uttermost, is able to give the command to the nerves that certain 

 muscles shall execute a certain motion. This interval varies in 

 different persons, and depends chiefly upon the degree of atten- 

 tion ; it varies also at different times in the case of the same 

 person. When the attention is lax, it is very irregular ; but 

 when fixed, on the contrary, very regular. The command travels 

 probably with the above velocity towards the muscle. Finally, 

 about T ~dth of a second passes after the receipt of the com- 

 mand before the muscle is in activity. In all, therefore, from 

 the excitation of the sensitive nerves till the moving of the muscle, 

 1£ to 2 tenths of a second are consumed. The measurements 

 are effected similarly to those on the frog. A slight electric 

 shock is given to a man at a certain portion of the skin, and he 

 is directed the moment he feels the stroke to make a certain 

 motion as quickly as he possibly can, with the hands or with the 

 teeth, by which the time-measuring current is interrupted. We 

 are therefore only able to measure the sum of the intervals above 

 indicated. When, however, the impression is caused to proceed 

 from different spots of the skin, some nearer to the brain and 

 others more distant, we change only the first member of the 

 above sum, that is, the velocity of propagation in the nerves. 

 At all events, we may, I think, assume that the duration of the 

 processes of perceiving and willing in the brain does not depend 

 upon the place on the skin at which the impression is made. I 

 must, however, confess that this is not a strictly proved fact ; it 

 can only be proved that the duration does not depend upon the 

 sensitiveness of the place of excitement, or on any particular 

 physiological relations between it and the moving muscle. Our 

 indication is rendered probable by the fact, that the numerical 

 values of the velocity of propagation, deduced from observations 

 in which the impression was received by the ear, the skin of the 

 face, the neck, the hands, the loins and the feet, exhibit a suffi- 

 cient agreement. It is found, for example, that intelligence 



